1. Field of the Invention
The present invention relates to a filter assembly and a communication apparatus used in the microwave band.
2. Description of the Related Art
A diversity-enabled CDMA (code division multiple access) cellular telephone system requires simultaneous transmission/reception communication. In an electric circuit for RF components in a cellular telephone terminal device, filter assemblies are typically formed of a combination of a shared antenna unit and a diversity-enabled reception filter. However, this requires a great number of filters, thus preventing the cellular telephone terminal device from being reduced in size.
In order to solve the above-described problem, as described in Japanese Examined Patent Application Publication No. 7-79204 and in Japanese Patent No. 2602121, a diversity-enabled communication apparatus has been suggested which incorporates a filter having two antenna terminals without increasing the number of filters.
However, there has been a problem associated with a reception filter of a communication apparatus of this type in that a different number of resonators are required depending upon whether the reception filter is connected to the first antenna (or a main antenna) or the second antenna (or a diversity antenna). Specifically, when connected to the first antenna, the reception filter requires one resonator more than when connected to the second antenna. For example, when connected to the second antenna, the reception filter is a band-pass filter having three resonators, while the reception filter is a band-pass filter having four resonators when connected to the first antenna. If the reception filter is optimally designed on the basis of the state when the filter is connected to the second antenna, the attenuation amount is excessively high and the insertion loss is high because the reception filter requires a greater number of resonators when connected to the first antenna.
If the reception filter is optimally designed on the basis of the state when the filter is connected to the first antenna, the attenuation amount is insufficient because the reception filter requires too few a number of resonators when connected to the second antenna. If the reception filter is optimally designed as a four-stage band-pass filter, the reception filter has a larger insertion loss and a larger size than a reception filter which is optimally designed as a three-stage band-pass filter.
Accordingly, the reception filter of the communication apparatus described in Japanese Examined Patent Application Publication 7-79204 and Japanese Patent No. 2602121 requires a different number of resonators and has different electric characteristics depending upon whether the reception filter is connected to the first antenna or the second antenna. It is thus difficult to optimize the filter characteristic. The first antenna serving as a main antenna and the second antenna serving as a diversity antenna have different capabilities, leading to an inconvenient communication apparatus.